Support e.g. `SINGULARITY_OPTIONS="--writable --fakeroot" eic-shell`

Use case: enable write access to the container with writable --fakeroot. This will also require that the user has write access to the directory in which the container is stored, so not on cvmfs.

install.sh

@@ -280,7 +280,7 @@ fi
 
 export ATHENA_PREFIX=$PREFIX/local
 export SINGULARITY_BINDPATH=$BINDPATH
-\${SINGULARITY:-$SINGULARITY} exec \${SIF:-$SIF} eic-shell \$@
+\${SINGULARITY:-$SINGULARITY} exec \${SINGULARITY_OPTIONS:-} \${SIF:-$SIF} eic-shell \$@
 EOF
 
   chmod +x eic-shell